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Session: Body - implant interfacing
Session starts: Friday 25 January, 13:00
Presentation starts: 13:15
Room: Lecture room 559

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Esther Sanchez Garza (Radboud university medical center, Radboud Institute for Health Sciences, Orthopaedic Research lab, Nijmegen, The Netherlands)
Christoph Schilling (Aesculap AG, R&D Biomechanical Research & Preclinical Evaluation, Tuttlingen, Germany)
Thomas Grupp (Aesculap AG, R&D Biomechanical Research & Preclinical Evaluation, Tuttlingen, Germany)
Nico Verdonschot (Radboud university medical center, Radboud Institute for Health Sciences, Orthopaedic Research lab, Nijmegen, The Netherlands)
Dennis Janssen (Radboud university medical center, Radboud Institute for Health Sciences, Orthopaedic Research lab, Nijmegen, The Netherlands)

Abstract:
INTRODUCTION: Cementless femoral total knee arthroplasty (TKA) implants use a press-fit or interference fit to achieve fixation. A greater interference fit, or thicker implant coating could lead to a superior fixation, but it could also introduce more damage to the bone during implantation. The primary stability is measured as the amount of relative displacement between the implant and the bone, which are known as micromotions. The purpose of the current study was to study the micromotions between two implant designs with different coating thickness. METHODS: A previous experimental set-up was used to test 6 pairs of human cadaveric femurs implanted with e.motion femoral components (Total Knee System, B. Braun) using a standard interference fit of 350µm and a novel interference fit of 700 µm. The specimens were subjected to the peak loads of gait (1960N) and squat (1935N), based on Orthoload dataset. Digital Image Correlation (DIC) was used to measure the micromotions as the shear component of the displacement at different regions of interest (ROIs); opening and closing of the implant was considered as the normal component of displacement. Univariate General Linear Models (GLMs) were created with design, loading, and ROI as fixed factors. RESULTS: No significant difference was found between the two interference fit (P=0.374). In contrast, loading had a significant difference (P=0.008) with micromotions being on average 10µ higher for squat. Loading also had a significant effect on gap opening (P<0.0001) with nearly 30µ higher for squat, whereas design was not a significant factor (P=0.9). DISCUSSION: In the current study, the primary stability of the same implant with two different interference fit was evaluated. The results demonstrate that increasing the coating thickness does not automatically influence the primary stability of a femoral TKA component. The exact relation between interference fit and primary implant stability still remains subject to debate and requires further evaluation, possibly utilizing a computational technique approach. SIGNIFICANCE: It is necessary to ensure a good primary stability between the bone and the implant for a better long-term fixation and reduce the risk of implant loosening.